FIG. 1 illustrates a relay or Relay Node (RN) 120 and User Equipments (UEs) 131 and 132 within the coverage area of an evolved Node B (eNB or eNode B) 110 in a wireless communication system 100. The RN 120 may forward data received from the eNB 110 to the UE 132 within the coverage area of the RN 120 and may forward data received from the UE 132 to the eNB 110. This eNB 110 may be referred to as a donor-eNB. The RN 120 may extend a high-rate area, improve communication quality at a cell edge, and support a communication service inside a building or beyond the coverage area of the eNB 110. In FIG. 1, a UE that receives a service directly from an eNB (hereinafter, referred to a Macro-UE or M-UE) such as the UE 131 coexists with a UE that receives a service from an eNB via an RN (hereinafter, referred to a Relay-UE or R-UE) such as the UE 132.
A radio link established between an eNB and an RN may be called a backhaul link. Specifically, a link from the eNB to the RN and a link from the RN to the eNB may be called a backhaul downlink and a backhaul uplink, respectively. A radio link established between an RN and a UE may be called an access link. Specifically, a link from the RN to the UE and a link from the UE to the RN may be called an access downlink and an access uplink, respectively.
Meanwhile, the quality of a radio link between an eNB and a UE may be degraded due to various factors. When the UE fails to receive a control signal from the eNB or the quality of a received signal is significantly degraded, this may be defined as a Radio Link Failure (RLF). To handle the RLF, the UE first identifies a problem at a physical layer and attempts to solve the physical layer problem. If the UE fails to recover from the physical layer problem, the UE may transmit a connection re-establishment request to the eNB, determining that an RLF has been detected.